JPH0678990B2 - Foreign object detection method and apparatus - Google Patents

Description

Description: TECHNICAL FIELD The present invention relates to a method and an apparatus for detecting a specific object on a sample, and more particularly to detecting foreign matter on a wafer having, for example, a fine circuit pattern on its surface. The present invention relates to a foreign matter detection method and apparatus suitable for doing so.

[Conventional technology]

A conventional foreign matter detecting device is described in, for example, Japanese Patent Laid-Open No. 59-65428, and a sample 2 on a stage 1 which reciprocates in the X direction as shown in FIG. The illustrated laser beam (hereinafter referred to as illumination light) 3 is irradiated, this pattern is magnified by the objective lens 4, and further magnified by the magnifying lens 5 to form an image 6. At this time, a device has been proposed in which the Fourier transform plane in the objective lens 4 is photographed by the field lens 7 to form an image 8 of the Fourier transform plane.

In this apparatus, an image 11 'created by illuminating the pattern 11 on the sample 2 with the illumination light 3 obliquely from above is used to irradiate the illumination light 3 as shown in the enlarged view of FIG. 4 (b). Pattern 11a, foreign material 11b, pattern corner 11c and pattern intersection 11d perpendicular to the direction (indicated by an arrow)
The light is scattered by. Therefore, the image 12 obtained by photographing this image 11 'with the field lens 7 has a pattern 12a, a foreign substance 12b, a pattern 12a perpendicular to the irradiation direction of the illumination light 3, as shown in an enlarged view of FIG. 4 (c). The corner portion 12c and the pattern intersection portion 12d etc. shine. When this image 12 is Fourier-transformed, the image 8 is an enlarged view of the image 13 in FIG.
As shown in, the light 13a from the pattern 11 becomes a straight line in the center, and the other light 13b is widely scattered to the periphery.
Then, when a light-shielding mask 14 shown in FIG. 4 (e) is applied to shield the light from the central vertical straight line, the information of the pattern 11 is shielded, and the final image formation 6 is an enlarged view of FIG. 4 (f).
As shown in No. 15, only the image 15a of the foreign matter and the image 15b of the pattern intersection portion or the pattern corner portion are formed.

Therefore, in the case of a large foreign substance, the signal thereof is larger than the signals of the pattern crossing portion and the pattern corner portion, and therefore the large foreign substance can be detected by detecting such a large signal. However, recent circuit patterns of semiconductor elements are becoming finer, and in such fine patterns, the signal at the pattern crossing portion or the pattern corner portion is often larger than the signal of the foreign matter.
With the above-mentioned device configuration, it is difficult to detect such foreign matter with a small signal.

Therefore, conventionally, for example, as described in Japanese Patent Laid-Open No. 59-6536, as shown in FIGS.
By comparing the detection images of the individual chips 22 and 23, the ones represented at the same position are judged as signals of the pattern, pattern corners or pattern intersections, and the signals of these chips are subtracted to delete them. If a difference signal remains, it is considered as a foreign object signal appearing at a different position, and an apparatus for detecting a foreign object has been proposed. Specifically, the image detected by the objective lens 24 passes through the endoscope 28 and is converted into an electric signal by the sensor 25, and an enlarged view thereof is shown in FIG. 5 (b).
First, the area 26 having the width W is stored as shown in FIG. Next, the currently detected image signal of the area 27 of the chip 23 is compared with the previously stored signal, and a different one is determined as a foreign substance. Incidentally, 29 in the figure is an illumination lamp.

FIG. 6 is a plan view showing the signal of the image detected in FIG. The screens 31 and 32 represent a part of the region 26 in the chip 22 and a part of the region 27 in the chip 23, respectively, and the screen 33 is the result of subtracting the signal of the screen 31 from the screen 32. Signals that commonly exist on screens 31 and 32 (310 =
321, 311 = 322) are pattern intersections / corners, and the signal 323 that does not exist in common is a foreign matter signal.
(Signal 323) can be detected.

In a semiconductor, since the pattern is repeatedly exposed for each chip, the pattern superposition state changes for each chip, and the three-dimensional shape of the pattern changes little by little. Fig. 7
Reference numerals 35 and 36 are enlarged views of a part of different chips in which the same pattern is drawn, and it can be seen that the lower layer patterns 37 and 38 and the upper layer patterns 39 and 40 have different displacement amounts.

FIGS. 8 to 10 show cross-sectional views 41 to 43 of the pattern and corresponding detection signal magnitudes 44 to 46 when this portion is obliquely illuminated. As shown in FIG. 8, in the case where the positional deviation d is small, the detection signal 44 is large because the step portion becomes steep, but
As shown in FIG. 10, when the positional deviation is large, the step portion is small and the pattern edge is smooth, so that the detection signal 46 is small. FIG. 9 shows an intermediate state between the two. Even with similar pattern intersections / steps like this,
The value of the detection signal changes greatly depending on the amount of positional deviation of superposition, and in some cases, even if oblique illumination is performed, scattered light is hardly generated, and the detection signal may not be obtained.

[Problems to be solved by the invention]

Since it cannot be said that such pattern intersections / corners generate scattered light in common between chips, it is not possible to accurately detect only a foreign substance by simply subtracting signals between chips as in the prior art. It had a difficult problem.

The object of the present invention is to solve the above-mentioned conventional problems, even if the intensity difference of scattered light is not similarly detected from the edge portion of the circuit pattern between chips and a grayscale difference signal based on the scattered light occurs, the circuit pattern The present invention provides a foreign matter inspection method and an apparatus therefor capable of correctly detecting only a true foreign matter without erroneously detecting the edge portion of the.

[Means for solving problems]

Since there is a difference in the amount of pattern registration misalignment between chips within the allowable range, it cannot be said that the pattern intersection / corner always produces scattered light in common at the same position between chips. Since there are more than 100 chips of the same type on the top, it can be considered that the same pattern intersection / corner in at least two of these chips may generate scattered light at the same position in common. Not natural. On the other hand, regarding foreign matter, the probability of foreign matter existing on the same coordinates in different chips can be considered to be zero, so scattered light does not occur on the same coordinates between chips. The present invention has been made paying attention to this fact, and compares the grayscale image signals sequentially detected based on the scattered light from each of the adjacent chips, and compares the grayscale difference signals indicating the grayscale difference with each other. If it is equal to or more than the value, it is sequentially extracted as a foreign matter candidate signal, and in synchronization with the extraction of the foreign matter candidate signal, if detected with a predetermined value or more for each of the grayscale image signal of the front chip and the grayscale image signal of the rear chip, This is regarded as the same pattern signal between chips, and this pattern signal is removed from the foreign substance candidate signal to detect a true foreign substance.

The characteristic features of the present invention will be listed below, and the present invention will be described in more detail.

First, the foreign matter detection method of the first invention will be described.

(1) Illuminating light from each of the microscopic regions showing the same pattern on each of the repeated chips on the sample from the vertical direction in the oblique direction inclined at a desired inclination angle from each of the microscopic regions sequentially illuminated. The scattered light of is received by the photoelectric conversion means, the grayscale image signals based on the scattered light of the chips are sequentially detected, and the sequentially detected grayscale image signals of the previous chip are sequentially stored in the storage means, and sequentially stored. When the grayscale difference signal indicating the grayscale difference between the grayscale image signal of the previous chip and the grayscale image signal of the subsequent chip that has been sequentially detected is sequentially calculated and the grayscale difference signal is equal to or greater than a predetermined value, this is used as the foreign object candidate signal. A predetermined value or more for each of the sequentially stored grayscale image signal of the preceding chip and the sequentially detected grayscale image signal of the subsequent chip, which are sequentially extracted and synchronized with the extraction of the foreign substance candidate signal. If issued, an foreign substance detecting method characterized by detecting a true foreign matter removing the pattern signal from said foreign object candidate signal is regarded to be the same pattern signal between this tip.

(2) A laser beam is used as the illumination light, the sample is a semiconductor wafer, and the detection target is a foreign substance on the wafer.

Next, the foreign matter detection device of the second invention will be described.

(1) An illumination optical system that illuminates illumination light from a slanting direction that is inclined at a desired slant angle from the vertical direction to each of microscopic regions that show the same pattern on each of repeated chips on a sample, and is illuminated by the illumination optical system. A detection optical system that receives scattered light from each minute region by photoelectric conversion means and detects a grayscale image signal based on each scattered light between chips, and a front chip detected by the photoelectric conversion means of the detection optical system Between the gray scale image signal of the previous chip stored by the first storage means and the gray scale image signal of the subsequent chip detected by the photoelectric conversion means of the detection optical system. When the gray level difference signal indicating the gray level difference is calculated and the gray level difference signal is equal to or greater than a predetermined value, the foreign particle candidate signal extracting means and the foreign particle candidate signal extracting means differ from each other. A predetermined value or more for each of the grayscale image signal of the previous chip stored in the first storage means and the grayscale image signal of the subsequent chip detected by the photoelectric conversion means of the detection optical system in synchronization with the extraction of the candidate signal. If it is detected by the above, it is regarded as the same pattern signal between the chips, the pattern signal is removed from the foreign substance candidate signal extracted by the foreign substance candidate signal extracting means, and the true foreign substance is detected. And a means for detecting the foreign matter.

(2) In the foreign matter detection device, the second foreign matter candidate signal extracting means stores the extracted foreign matter candidate signal.
It is characterized by having a storage means of.

(3) In the foreign matter detection device, as the true foreign matter detection means, the grayscale image signal of the previous chip stored in the first storage means is binarized with a predetermined threshold value and converted into a binarized signal. A binarizing unit 1 and a grayscale image signal of the post-chip detected by the photoelectric conversion unit of the detection optical system is binarized by a predetermined threshold value and converted into a binarized signal.
And the binary value obtained from the first binary value conversion means.
It is characterized by further comprising: a logical product circuit for taking a logical product of the binarized signal and the binarized signal obtained from the second binarization means to form the pattern signal.

(4) The foreign matter detecting device is characterized in that it has third storage means for storing a binarized signal as the first binarization means.

[Action]

As described above, when comparing the grayscale image signals based on the scattered light of each chip repeatedly formed on the sample such as the wafer and calculating the grayscale difference signal indicating the grayscale difference between them, FIG. ~ As shown in Fig. 10, due to the difference in the edge shape of the pattern or the positional deviation, there is a difference in the intensity (shading) of the scattered light generated from the edge of the pattern between chips, and it is possible to identify whether it is a foreign substance or the edge of the pattern. It will not be possible. However, since the grayscale image signal based on the scattered light is detected from the edge of the pattern of each chip, if each grayscale image signal is binarized with a low threshold value, for example, a signal corresponding to the edge of the pattern is detected. If the detected signal is detected by both chips, it can be regarded as an edge signal of the pattern.
Therefore, in each of repeated chips on a sample such as a wafer, each of minute regions showing the same pattern is sequentially irradiated with illumination light from an oblique direction inclined at a desired inclination angle from the vertical direction, and each minute region illuminated sequentially. The scattered light from the area is received by the photoelectric conversion means, the grayscale image signals based on the scattered light of the chips are sequentially detected, and the sequentially detected grayscale image signals of the previous chip are sequentially stored in the storage means, and sequentially stored. If a gray level difference signal indicating a gray level difference between the gray level image signal of the preceding chip and the gray level image signal of the subsequent chip that are sequentially detected is sequentially calculated and the gray level difference signal is equal to or greater than a predetermined value, this is a foreign substance candidate. The signals are sequentially extracted as a signal, and in synchronization with the extraction of the foreign-matter candidate signal, a predetermined value is given to each of the sequentially stored grayscale image signal of the previous chip and the sequentially detected grayscale image signal of the subsequent chip. When it is detected with a value or more, it is regarded as the same pattern signal between chips and the true foreign matter can be detected by removing the pattern signal from the foreign matter candidate signal. .

〔Example〕

An embodiment of the present invention will be described below with reference to FIG. The figure shows an apparatus for detecting foreign matter on a patterned product wafer.

A laser beam 52 (light source is not shown) is applied to a small area 51 on the wafer 50.
Then, the small area is enlarged by the objective lens 53, and the image pickup element 56 is detected through the field lens 54 and the magnifying lens 55. A light-shielding mask 57 corresponding to the light-shielding mask 14 described in FIG. 4 is provided at the position of the real image (conjugate image) on the Fourier transform surface of the objective lens 53 to remove the pattern information on the wafer. In this state, the XY stage 58 is moved in the X direction.

First, an electric circuit for obtaining foreign object candidate coordinates will be described. First, change the write-side selector switch 81 to the solid line state,
The small area 71 in the chip 61 is detected and stored in the first memory 82. Next, the write side changeover switch 81 and the read side changeover switch 83 are changed over to the state of the broken line, and the detection signal of the small area 72 in the chip 62 taken out from the signal 84 and the first memory taken out from the signal line 85. Signal in 82 (Detection signal of small area 71)
And are subtracted by the operator amplifier 86. The changeover switches 81 and 83 are changed over at high speed, the signal in the first memory 82 is sent out, and the signal detected by the image pickup device 56 is transmitted to the signal line 84 in the area where the blank occurs and the first line is also transmitted. Memory 82
Can be stored in This is the detection signal of the small area 72, and is used for subtraction with the detection signal of the small area 73 to be detected thereafter.

The signal passed through the operator amplifier 86 is
It is binarized at 87, and the result is stored in the fourth memory 88 together with the coordinates in the chip and the code when binarized. Here, the coordinate of the signal in the chip is obtained by obtaining the X coordinate by the position detector (not shown) attached to the XY stage 58 and the Y coordinate from the pixel number of the image pickup device 56. Has become.

The signal processing situation during this period will be schematically described with reference to FIG. FIG. 2A shows a detection signal in a small area in each chip, and FIG. 2B shows the subtraction result by the operator amplifier 87.
It is a value and is expressed with a sign. FIG. 3 shows foreign substance candidate coordinates (A) showing the contents written in the memory 88.
Is. The subtraction result shown in FIG. 2B is written with the coordinates in the chip. The symbol − indicates that the foreign substance candidate is present on the right side chip (here, chip No. 62), and the symbol + indicates that it is present on the left side (here, chip No. 61). As a matter of course, the left and right chips have the same signal (in the case of chip Nos. 61 and 62, the signal 610 = 62).
0) is gone.

Referring back to FIG. 1 again, an electric circuit for creating the pattern intersection / corner coordinate map will be described. This is to create a map by regarding the case where a detection signal is generated at least twice at the same place in a chip as a pattern intersection / corner.

The signal of the memory 82 (where the detection signal of the small area 71 is stored) is binarized by the operator amplifier 91 and stored in the second memory 92. At the same time, the detection signal of the adjacent small area 72 is binarized by the operator amplifier 93, and the result is stored in the third memory 94 by taking the logical product with the contents of the memory 92. The signal processing situation during this period will be described with reference to FIGS. 2 (c) and 2 (d). FIG. 2C shows the contents of the memory 92.
All the binarized results detected by 56 are stored. Second
Figure (d) shows the contents of the third memory 94,
The detection signal is stored only when the binarized signal is generated at least twice in the same coordinate. That is, the third memory 94 stores signals at the pattern intersection / corner portion due to the pattern in the chip.

Returning to FIG. 1 again, the information (pattern crossing portion / corner portion) (B) accumulated in the third memory 94 from the information (foreign particle candidate coordinates) (A) accumulated in the fourth memory 88 is changed. The information 95 of the coordinates of the foreign matter is obtained by the removal.

〔The invention's effect〕

According to the present invention, it is possible to distinguish a foreign material on a patterned sample from a pattern, and it is possible to detect a minute foreign material.

When the present invention is not used, only foreign matters that generate scattered light larger than scattered light generated at pattern intersections / corners can be detected, and in the case of a wafer having a multilayer pattern formed, it is 3 to 4 μm or more. Only foreign objects can be detected. If the pattern crossing portion / corner portion is removed by using the present invention, a 1 μm foreign substance can be detected even in the case of a wafer having a multilayer pattern formed, and the foreign substance detection performance is remarkably improved.

When the present invention is applied, memories such as 82, 92, 94 shown in FIG. 1 are required. Assuming that the width of the small region 72 is 1.5 mm and the length is 10 mm in FIG. 1, the area is 15 mm ² . If the pixel (minimum unit for image detection) is 5 μm square, the number of pixels is 0.6 × 1
0 ⁶ and will, Using LSI memory 1M bits ⁽¹⁰⁶ bits) can be sufficiently recorded one by. The first memory 82 needs to store an analog signal, and assuming that one signal is digitized into 64 gradations and stored, 6 pixels per pixel are stored.
Bits or 6 1Mb LSIs are required. Further, since the first and second memories 92 and 94 only need to store the binarized signal, one 1 Mb LSI is required for each. Even if the above is summed up, it is possible to perform all the above processing by using eight 1 Mb LSIs, and it is possible to achieve excellent performance with such a small electric circuit.

〔The invention's effect〕

According to the present invention, the grayscale image signal of the front chip and the grayscale of the rear chip based on the scattered light obtained from the minute area showing the same pattern in each of the chips repeatedly formed with the fine circuit pattern on the sample such as the wafer. There is an effect that a true foreign substance can be detected with high reliability without erroneous detection from a grayscale difference signal indicating a grayscale difference between the image signal and the image signal.

[Brief description of drawings]

FIG. 1 is a foreign matter detection device on a patterned wafer and a detection signal processing circuit according to an embodiment of the present invention, and FIG. 2 is an explanatory view of a processing situation inside a detection signal processing circuit according to the embodiment of the present invention. FIG. 3 is an explanatory view of a format when the contents of FIG. 2 (b) are stored in a memory, FIG. 4 is a configuration diagram of a conventional foreign matter detection apparatus on a patterned wafer, and FIG. FIG. 6 is an explanatory view of the operation principle of the foreign matter detection device on a wafer, FIG. 6 is an explanatory view of extracting only foreign matter from a foreign matter candidate map, and FIG.
The figure is an illustration of misregistration due to overlay error during semiconductor pattern exposure, Fig. 8 shows the pattern cross section and detection signal when the misregistration is small, and Fig. 9 is the same when the misregistration is medium. FIG. 10 is a diagram showing the pattern cross section and the detection signal, and FIG. 10 is an explanatory diagram showing the pattern cross section and the detection signal when the positional deviation is also large. In the figure, 1 ... XY stage, 2 ... Sample wafer 4 ... Objective lens, 50 ... Wafer 51 ... Small area, 52 ... Laser light 53 ... Objective lens, 54 ... Field lens 55 ... Enlarge Lens, 56 …… Image sensor 61, 62, 63 …… Chip, 71, 72, 73 …… Small area 81 …… Write side selector switch 83 …… Read side selector switch 82, 92, 94, 88 …… Memory 86 , 87,91,93 …… Operator amplifier

Claims (6)

[Claims] 1. A microscopic region showing the same pattern on each of repeated chips on a sample is sequentially illuminated with illumination light from an oblique direction inclined at a desired inclination angle from a vertical direction, and each microscopically illuminated microscopic region is illuminated. The scattered light from the area is received by the photoelectric conversion means, the grayscale image signals based on the scattered light of the chips are sequentially detected, and the sequentially detected grayscale image signals of the previous chip are sequentially stored in the storage means, and sequentially stored. If a gray level difference signal indicating a gray level difference between the gray level image signal of the preceding chip and the gray level image signal of the subsequent chip that are sequentially detected is sequentially calculated and the gray level difference signal is equal to or greater than a predetermined value, this is a foreign substance candidate. A predetermined value for each of the sequentially stored grayscale image signal of the preceding chip and the sequentially detected grayscale image signal of the succeeding chip, which are sequentially extracted as signals and are synchronized with the extraction of the foreign substance candidate signal. If it is detected above, the foreign matter detecting method characterized by detecting a true foreign matter removing the pattern signal from said foreign object candidate signal is regarded to be the same pattern signal between this tip. 2. The foreign matter detecting method according to claim 1, wherein laser light is used as the illumination light. 3. An illumination optical system for irradiating each of minute regions showing the same pattern on each of repeated chips on a sample with an illumination light from a vertical direction at a desired inclination angle, and an illumination optical system. A detection optical system that receives scattered light from each illuminated minute area by photoelectric conversion means and detects a grayscale image signal based on each scattered light between chips, and a photoelectric conversion means of the detection optical system. First storage means for storing the grayscale image signal of the front chip, the grayscale image signal of the front chip stored in the first storage means, and the grayscale image signal of the rear chip detected by the photoelectric conversion means of the detection optical system. And a foreign matter candidate signal extracting means for calculating a grayscale difference signal indicating a grayscale difference between A predetermined value for each of the grayscale image signal of the front chip stored in the first storage means and the grayscale image signal of the rear chip detected by the photoelectric conversion means of the detection optical system in synchronization with the extraction of the object candidate signal. When detected above, it is regarded as the same pattern signal between the chips, the pattern signal is removed from the foreign particle candidate signal extracted by the foreign particle candidate signal extraction means, and a true foreign particle is detected. And a means for detecting foreign matter. 4. The foreign matter detecting device according to claim 3, further comprising second storage means for storing the extracted foreign matter candidate signal as the foreign matter candidate signal extracting means. 5. The first foreign matter detecting means for binarizing the grayscale image signal of the previous chip stored in the first storing means by a predetermined threshold value and converting it into a binarized signal.
The grayscale image signal of the post-chip detected by the value conversion means and the photoelectric conversion means of the detection optical system is set to 2 with a predetermined threshold value.
Second binarizing means for converting the value into a binarized signal, a binarized signal obtained from the first binarizing means, and a binarized signal obtained from the second binarizing means. 4. The foreign matter detection device according to claim 3, further comprising: a logical product circuit that obtains the logical product of the two to form the pattern signal. 6. The foreign matter detection device according to claim 5, further comprising a third storage means for storing a binarized signal as the first binarization means.